With the use of any form of electrical appliance, there is a need to electrically insulate conductors. Conductors need to be wrapped in a insulating tape to prevent electric discharge. An example of this is shown in FIG. 1. Illustrated here is a coil 13, comprising a plurality of turns of conductors 14. Each turn of the conductor 14 consists essentially of a copper bar or wire wrapped with a turn insulation 15. The turn insulation 15 is prepared preferably from a fibrous sheet or strip which is typically impregnated with a resin. The turn insulation 15 may be not adequate alone to withstand the severe voltage gradients that will be present between the conductor and ground when the coil is installed in a high-voltage generator. Therefore, ground insulation for the coil is provided by wrapping one or more layers of composite mica tape 16 about the turn 14.
Such a mica-tape 16 comprises a pliable backing sheet 18 of, for example, poly-ethylene glycol terephthalate or glass fabric mat, having a layer of mica flakes 20 bonded to a resin. The tape 16 may be applied half lapped, abutted or in any other suitable manner. Generally, multiple layers of the mica tape 16 are wrapped about the coil with sixteen or more layers generally being used for high voltage coils. The number of layers may be decreased depending on the power of the generator and the effectiveness of the insulator in both its abilities to insulate electrically and conduct heat. To impart better abrasion resistance and to secure a tighter insulation, a wrapping of an outer tape 21 of a tough fibrous material, for example, glass fiber, asbestos or the like may be applied to the coil.
Therefore, what is referred to as insulating tape is actually composed of multiple layers of tape that have different properties. The inner most layer is referred to as the groundwall insulation. Wrapped around this is the conductive layer. The conductive layer provides a low resistance and doesn't allow voltage to be present between the outer coil surface and the core. As will be discussed below, in order to function properly as a conducting electrode, the conductive layer needs to be in firm contact with both the groundwall insulation and the generator assembly in which the wrapped conductor (coil) is inserted.
As mentioned, the insulating tape is generally impregnated with a resin to improve many of its overall properties. There are many methods of coating materials with epoxy resins and then curing the product. One such method is vacuum pressure impregnation (VPI). This method is used on devices such as stator conductor coils. A mica/glass insulating tape is applied to the coils, then the coils are placed in a vacuum vessel and a vacuum is applied. After a period of time, resin is admitted to impregnate the coils. Pressure is applied to force the resin in and minimize voids, which will affect conductivity. After this is completed, the coils are heated to cure the resin. The resin may contain an accelerator or the tape may have one in it. A variation of this, global VPI involves the process where dry insulated coils are wound, and the then whole stator is vacuum pressure impregnated rather than the individual coils.
The wrapped conductor is then placed into machinery such as a generator. FIG. 2 shows one embodiment of a generator in cross section. The generator comprises a metal armature or rotor 28 having slots 22 therein, containing insulated coils 23, surrounded by a metal stator 24 having slots 25 therein about the stator circumference at 26. The stator slots contain insulated coils 27.
If the conductor is not secure against the generator assembly, electric discharge will result. This adversely affects the performance of the machinery, and also causes cumulative damage to the generator, conductor and insulation tape.
In order to prevent such a discharge, the conductive layer of the insulating tape itself needs to be made of at least two layers, which are referred to as the outer conductive layer and the inner conductive layer. Note that both of these sub layers may themselves be composed of multiple layer depending on need. The outer layer of conductive tape will be in firm contact with the generator assembly, while the inner conductive layer will be in firm contact with the groundwall insulation. This, however, creates a problem, since the conductor and the generator assembly often have minor movements independent of one another due to such things as heating and vibration. This is referred to as a difference of movement. If the outer conductive layer of the insulating tape is in firm contact with the generator assembly, and the conductor moves independently of the assembly, stresses are created on the insulating tape.
These stresses cause the tape to tear, ruining the insulation around the conductor coil. To prevent this, machinery is run at below desired level to prevent movements due to vibration and temperature.
An alternate solution is to provide a slip layer in the insulating tape. This slip layer is sandwiched between an inner conductive layer, which is in contact with the conductor, and the outer conductive layer, which is in contact with the generator assembly. The slip layer consists of a mica-filled tape that is interwoven with a conductive tape, as shown in FIG. 3. The mica-filled slip layer consist primarily of large mica flakes, generally larger than those used in other insulating layers. Therefore they are not well bonded together and can slip relative to each other. A conductor 13 is wrapped with a groundwall insulation layer 43 and then a inner conductive layer 44 followed by a mica-filled tape 41 that has a conductive interweave 42 woven horizontally along one side. This slip layer allows for a minor difference of movement between the inner 44 and outer 46 conductive layers, without causing any tears or damage to the tape.
This solution, however, is not with out its own problems. One concern is that the slip layer, because of being a mica-filled tape, is extremely delicate. This means that the slip layer, and often the entire insulating layer, has to be wound around the conductor by hand, rather than using more efficient machinery. Because the mica-filled tape has large flakes, it is delicate and there for is more susceptible to handling damage. Also, the amount of slip that the slip layer allows for is only moderate. Therefore the difference of movement between the conductor and generator assembly cannot be too significant. Further, the mica-filled tape is on average 11.5 mils thick (0.3 mm). Since the assembly needs to produce an overlap of the tape, this increases the overall insulating tape thickness to 25 mils (0.65 mm) per side of the conductor coil. This added thickness not only adds to the overall size of related machinery, but also reduces efficiency in properties such as thermal conductivity.
The mica-filled tape, however, has not been able to be replaced because of the nature of the mica-flakes. They provide the needed dielectric strength. They are also porous to the resins used in impregnating the insulating tape. Without proper porosity, the inner conductive layer will not saturate properly, thus ruining the properties of the insulating tape.
What is needed is a slip layer that is thinner, more resilient and that allows a greater slip tolerance, while maintaining or improving upon the dielectric properties and porosity of the prior art.